Capacitor Start on CSCR Motor Turn Off Circuit

F

Thread Starter

float

I have a capacitor start/capacitor run 2KW motor ([email protected]). I have a small board with a rc circuit (for 2s timming - enough for motor start) activating a relay that connects/disconnects the start capacitor. I'm trying to replace the relay. Are there any circuits to replace the relay with triacs/thyristors on this configuration? I've tried a "simple replace" with a triac (40A & 1200V) and a rc snubber. Triac turns on the start capacitor, but when i remove the signal from the gate (by the optocoupler, 2s later) the triac blows. I believe that its some kind of dc/dt or di/dt problem, but on this configuration (inductance from the motor plus 2 capacitors, i don't know where to start). Thanks in advance for any hints.
 
You are connecting the triac to the starting winding circuit.
Due to very low powerfactor the current will be very large (the actual current after the summing up of the starting and running winding currents will be much much smaller). A 40 A triac may not be able to handle this inrush current. They can get damaged in a few cycles.
Since the starting capacitor and starting winding are connected in series, the voltage across the triac after opening the circuit will also be large and can kill the triac.
Commercial triacs have very low dI/dT rating and will fuse or blow the device. These are not meant for low powerfactor applications.
You can connect two HV high di/dt thyristors back to back and try.
Happy safe experimenting though relays are lot cheaper.
regards,
Sekar
 
Top